A melt spinning process is a one-step process to convert molten alloy into a solid foil. In this process, a metal ingot is melted in an enclosed crucible producing molten metal. The bottom of the crucible has an orifice or a series of orifices spaced across a rotating quenching wheel which lies directly beneath the crucible. The molten metal in the crucible is ejected as a stream of molten metal through the orifice(s) onto the rotating quenching wheel. This stream forms a puddle from which thin foil is continuously solidified at the interface of the puddle and the rotating wheel. The molten metal is ejected from the orifice(s) of the crucible by pressure caused by the hydrostatic head height of the molten metal and by gas overpressure. The gas producing the overpressure is injected through an opening at the top of the crucible over the molten metal in the crucible.
The dimensions of the finished metal-foil product are determined by both the dimension and flow rate of the stream of molten metal which is ejected through the orifice(s) of the crucible and by the characteristics of the rotating quenching wheel, i.e., its size, rotational mean interface speed, temperature and composition. The cross-sectional dimensions of the stream of molten metal are determined by the dimensional characteristics of the orifice(s) at the bottom of the crucible. Generally, the width of the foil is about the same as the long dimension of a slotted orifice or the length of a spaced array of circular orifices at the bottom of the crucible aligned perpendicular to the direction of movement of the rotating wheel.
The effective ejection pressure of the molten metal is one of the major factors determining the thickness of the metal foil produced. In other words, the foil thickness in a continuous run will vary if the effective ejection pressure is not kept at a constant level. This effective ejection pressure P.sub.eject, is composed of the sum of the overpressure produced by the inert gas applied to the melt, P.sub.gas, and the pressure caused by the hydrostatic head height of the melt itself, P.sub.melt i.e., P.sub.eject =P.sub.gas +P.sub.melt.
During the course of a run, P.sub.melt will decrease due to the continuous reduction of the head height of an unrecharged molten metal source. If this decrease in P.sub.melt is not compensated for, P.sub.eject will also decrease which will cause a continual decrease in the thickness of the metal foil from the beginning to the end of the run. Therefore, in order to compensate for the continuous decrease of P.sub.melt, a means for a controlled increase of P.sub.gas is needed in order to maintain a constant effective ejection pressure. Commercially available pressure sensors have not been designed to withstand immersion into hot molten metals, which for the case of AMS 4778 Ni-base alloy is about 1100.degree. C. The conventional approach is to first calculate or estimate the time rate of change for P.sub.melt. When ##EQU1## estimated or calculated then a controlled increase of P.sub.gas using available commercial instruments is attempted to maintain the instantaneous rate for remains zero. But, the relationship between P.sub.gas and P.sub.melt depends on the initial height and density of the molten metal charge, the size and shape of both the crucible vessel and the orifice. In other words, ##EQU2## is a variable that must be calculated for every different kind of run in order to program the increase in P.sub.gas to hold P.sub.eject constant. This is a complicated, tedious approach which has not reliably produced metal foil with end to end thickness uniformity.